Recording materials which utilize electron-donating dye precursors (color former) and electron-accepting compounds (color developer) are well known for use as pressure-sensitive recording paper, heat-sensitive paper, photo- and pressure-sensitive recording paper, electric heat-sensitive recording paper, and the like.
Such recording materials are described in detail, e.g., in British Pat. No. 2,140,449, U.S. Pat. Nos. 4,480,052 and 4,436,920, Japanese Patent Publication No. 23922/85, Japanese Patent Application (OPI) Nos. 179836/82, 123556/85 and 123557/85 (the term "OPI" as used herein refers to "published unexamined Japanese patent application"), and so on.
A recording material should have properties of (1) producing developed color image of sufficiently high density at a satisfactory speed, (2) generating no fog, (3) ensuring sufficient fastness to the developed color image it produces, (4) ensuring an appropriate hue to the developed color image it produces, (5) having an aptitude for copying apparatuses, (6) having a high signal to noise ratio (S/N ratio), (7) ensuring sufficient chemical resistance to the developed color image it produces, and so on. However, recording materials which meet perfectly all of these essential properties have not yet been obtained.
In particular, heat-sensitive recording materials have made remarkable progress in recent years. However, they have defects such as that they generate fog by contact with solvents or the like, and the developed color images they produce cause discoloration or decolorization upon contact with fats and oils, chemicals, finger tips, and so on. Accordingly, color development occurs in white background areas, or discoloration or decoloration occurs in developed color image areas when a conventional heat-sensitive recording material happens to come into contact with stationary writing materials or office supplies, such as a water-base ink pen, an oil-base ink pen, a fluorescent pen, vermilion inkpad, adhesives, paste, a diazo developer, etc., or cosmetics such as hand cream, milky lotion, etc., which can cause significant damage to the commodity value. There has recently been a striking growth in the demand of heat-sensitive recording materials for use as POS labels, and the market demand for heat-sensitive recording materials having chemical resistance is increasing considerably.